1. Field of the Invention
The present invention relates generally to a color-separation prism assembly, and more particularly relates to a color-separator prism with adjustable path lengths.
2. The Prior Art
A number of color-separation prism designs have been in widespread use. A well-known design is the so-called "Philips" prism assembly. A lateral cross-section of a prior art Philips prism assembly is shown in FIG. 1. A Philips prism assembly is formed of three individual prisms, a first prism 10, a second prism 12 and a third prism 14, each of which has certain flat, optically polished surfaces. First prism 10 has optical surfaces 16, 18 and 20. Second prism 12 has optical surfaces 22, 24 and 26. Third prism 14 has optical surfaces 28 and 30. In addition to their optical finish, surfaces 18 and 22 (or 28) have multi-layer dichroic coatings that cause them to have graded reflection versus wavelength to make filter functions that are approximately color matching functions. Some unwanted far-off-axis rays may find one-bounce or three-bounce paths to the exit surface of the first prism 10. A flare-stop notch 32 is provided in first prism 10 to block such unwanted light paths that would otherwise cause flare without blocking desired ray paths, as is readily understood by persons of ordinary skill in the art.
The Philips prism assembly operates in the following manner: a ray 40 of light from the incoming image enters first prism 10 at normal incidence through surface 16. Most of the blue light in the ray (wavelengths shorter than about 500 nanometers) is reflected to form ray 42, while the remainder of the light is transmitted to form ray 44. Ray 42 is totally internally reflected from surface 16 to form ray 46, which passes through surface 20 at normal incidence to form the blue output of the assembly. Most of the red light in the ray 44 (wavelengths greater than about 580 nanometers) is reflected off surface 22 to form ray 48, which is totally internally reflected from surface 24 to form ray 50, which passes through surface 26 at normal incidence to form the red output of the assembly. The remainder of the light passes into third prism 14 to form ray 52, which passes through surface 30 at normal incidence to form the green output of the assembly. Other rays entering at non-normal incidence follow corresponding nearby paths.
It is understood that the wavelengths quoted are typical of industrial practice, and can vary somewhat in different applications, as is well understood by those of ordinary skill in the art. Such skilled persons are aware that the crossover wavelength (the wavelength of transition between transmission and reflection) of a typical dichroic coating is insensitive to polarization at normal incidence (0 degree angle-of-incidence, ray perpendicular to surface), but becomes increasingly sensitive to polarization as the angle of incidence is increased. A first advantage of the Philips prism assembly is based on this fact. Both surface 18 and surface 22 are operating at less than 30.degree. angle of incidence. For this reason, the crossover wavelength for polarizations in and perpendicular to the page are nearly equal, and excellent color separation results. A second advantage of the Philips prism assembly is that the green ray 52 exits the assembly having encountered no reflections, while red ray 50 and blue ray 44 exit the assembly having encountered two reflections so that none of the exiting rays are mirrored by having an odd number of reflections. Another advantage of the Philips prism assembly is that the individual prisms can be slid relative to each other to adjust the lengths of the three optical paths to be the same.
The disadvantages of a Philips assembly stem from the total internal reflection of ray 48 on surface 24 to form ray 50. The requirement of total internal reflection from surface 24 requires an air gap 54 between the two surfaces. This air gap 54 must be fairly uniform and securely held, and must be protected from moisture accumulation. The two-reflection paths of rays 44 and 50 also lengthen the optical length to over 2 times the width of an exit face, which requires the use of lenses with a longer back working distance.
Additionally, the third prism 14 of the Philips prism assembly has limited slide movement along the second prism 12 toward the first prism 10, since the surface 18 of the first prism 10 creates an obstruction. In order to increase the adjustable optical path length to the output faces for the prism assembly, the third prism 16 needs to be able to traverse further across the second prism 14 toward the first prism 10 than is possible in the prior art.